For configuration on ASA ,
http://www.cisco.com/en/US/docs/security/asa/asa72/configuration/guide/site2sit.html
For configuration on the Linksys router
http://www.homenethelp.com/vpn/router-linksys.asp
The options on the VPN page may be a bit different depending on the firmware.
Make sure you have the same pre-shared key , Encryption, Authentication settings on both sides.
Also, on the Linksys box,
Under Remote Secure Group > select Subnet and then provide the LAN IP subnet that is behind the ASA.
Under Remote Security Gateway > select IP address and provide Public IP of the ASA box.
PFS > keep it disabled.
Hope this helps.